1. Field of the Invention
The present invention relates to semiconductor devices such as diode and transistor, and more particularly, to a semiconductor device having semiconductor crystal epitaxially grown in narrow grooves.
2. Description of the Related Art
FIG. 17 is a plan view of a conventional diode device 101, and FIG. 18 is a sectional view taken along the line P—P in FIG. 17. For the ease of illustration, FIG. 17 does not show a thermal oxide film, a PSG film, and an anode electrode that will be described later.
The diode device 101 has an N-type silicon substrate 111. An N-type epitaxial layer 112 is formed on the surface of the silicon substrate 111.
The surface of the epitaxial layer 112 is provided with ring-shaped holes and oblong holes in plane view. Herein, three ring-shaped holes and three oblong holes are provided. The ring-shaped holes are arranged concentrically at predetermined intervals, and the oblong holes are arranged within the inner circumference of the ring of the innermost ring-shaped hole.
A semiconductor layer formed by epitaxial growth and containing a P-type impurity is filled within the ring-shaped holes and oblong holes. The oblong holes filled with the semiconductor layer form withstanding voltage portions 1251 to 1253, and the ring-shaped holes filled with the semiconductor layer form guard ring portions 1271 to 1273.
A thermal oxide film 114 and a PSG film 115 are formed in that order on the surface of the epitaxial layer 112 including inner surface of holes. An anode electrode 118 of a thin metal film is arranged on the PSG film 115. The thermal oxide film 114 and the PSG film 115 are provided with an opening in identical positions. The edge of the opening is denoted by the broken line 162a in FIG. 17. The epitaxial layer 112, the withstanding voltage portions 1251 to 1253, and the innermost guard ring portion 1271 are exposed at the bottom of the opening. These exposed parts are in contact with the bottom of the anode electrode 118. The part of the anode electrode 118 in contact with the epitaxial layer 112 form Schottky junction portions 131. The part of the anode electrode 118 in contact with the withstanding voltage portions 1251 to 1253 and the innermost guard ring portion 1271 form ohmic junction portions 1301 to 1303.
In the above-described diode device 101, when a negative voltage for the anode electrode 118 is applied to a cathode electrode 119, the Schottky junction portions 131 between the anode electrode 118 and the epitaxial layer 112 are forward biased, which allows current to flow from the anode electrode 118 to the cathode electrode 119.
Conversely, when a positive voltage for the anode electrode 118 is applied to the cathode electrode 119, the Schottky junction portions 131 between the anode electrode 118 and the epitaxial layer 112, and the PN junctions between the withstanding voltage portions 1251 to 1253 and the innermost guard ring portion 1271 and the epitaxial layer 112 are reverse biased, and current is kept from flowing. In this state, a depletion layer is expanded horizontally from the PN junctions toward the epitaxial layer 112.
As the depletion layer expands, the epitaxial layer 112 positioned between the innermost guard ring portion 1271 and the withstanding voltage portions 1251 to 1253 and between the guard ring portions 127n and 127n+1 adjacent to each other is entirely depleted. At the same time, the guard ring portions 1271 to 1273 and the withstanding voltage portions 1251 to 1253 are all depleted inside, in other words, the part within the inner circumference of the ring of the outermost guard ring portion 1273 is entirely depleted. Electric field concentration that would occur in an area without a depletion layer does not occur, so that the breakdown voltage improves.
However, in the conventional diode device, breakdown occurs before the part within the inner circumference of the ring of the outermost guard ring portion 1273 is entirely depleted.